By Stefan Schroeter
Contributing Author, Cornerstone
An increasingly urbanized global community affords greater opportunities for the most efficient means to extract energy from coal and other fuels: combined heat and power (CHP) plants. CHP is not new. Some of the world’s first power plants were CHP facilities and they continue to be deployed globally today. Plant size, electricity output, and heat provided are site specific and the electricity and heat output can vary throughout the year as more heating is needed during cooler months. Germany is an example of a country that has been relying successfully for decades on a mix of large and small CHP facilities, many of which are coal fired.
For generating electricity at the highest overall fuel usage efficiency, CHP plants are unsurpassed. While the most modern simple-combustion coal-fired power stations deliver a maximum efficiency of 46%,A much of the energy in the fuel is not effectively utilized and thus leaves the power plant as waste heat. To get the most energy out of fuels, it therefore makes sense to consider the supplementary use of waste heat from power generation. Through careful planning and collaboration with urban centers, operating a CHP plant can result in 90% of the fuel’s energy being productively utilized.
When supplying municipalities with energy, it is clearly preferable to locate CHP plants as close as possible to, or even within, city limits. Their capacity and performance can be adapted to the heating demand of the service area. Waste heat from the power plant, which might otherwise be released through the stack or cooling devices, can instead be used to heat water. The combined heat and power cogeneration mode insures that the fuel will be used at high overall efficiencies of up to 90%. The heated water can then be conveyed over a short distance to the urban heating network. As a result, the required investments in existing district heating connections remain moderate.
In almost all parts of Germany, smaller CHP plants can be an effective fit for energy utilization. While large, modern coal-fired power plants provide relatively high electrical efficiencies, CHP facilities of varying sizes are able to achieve high fuel use efficiencies and can be suited to meet the needs of both large and small urban areas.
In Germany, large, high-efficiency coal-fired power plants are often sited in the vicinity of hard-coal and lignite surface mines or at coal ports to avoid transport costs. CHP is only a limited option for these plants as large cities are too distant to justify the cost of overland heating circuits. In addition, the immense quantities of waste heat produced by a large-scale power plant far exceed the thermal requirements of many cities and industrial complexes.
For these reasons, the fuel use efficiency of such power plants is only marginally improved by municipal or industrial heat deliveries, while the electrical efficiency can slightly decrease.
Four German coal-fired CHP plants are chronicled in this article. These plants were selected because three of them represent typical facilities found throughout Germany that differ from each other in terms of their size and fuel use efficiency. The remaining plant, Chemnitz, is a rare example of a municipal lignite-fired plant from the days of the German Democratic Republic that has survived to continue providing heat and power today. The operating information for these four plants is summarized in Table 1.
LIPPENDORF LIGNITE-FIRED PLANT PROVIDES ELECTRICITY AND DISTRICT HEAT TO LEIPZIG
Operated by the Swedish state-owned energy corporation Vattenfall, the advanced-technology Lippendorf lignite-fired power plant in Saxony, Germany, has provided utility heating since 2000. The CHP plant configuration has an electricity capacity of 1782 MW, generated at an electrical efficiency of 41.7%. In 2014, Lippendorf provided up to 330 MW of heat, primarily to the Leipzig Stadtwerke city utility service area. Heated water is transported from the plant through a 15-km insulated underground pipeline. Under this arrangement, the municipal utility is able to cover half the district heating requirements in the city of 550,000 inhabitants. A significantly lesser 1.6% of the heated water is routed from Lippendorf to the nearby municipalities of Böhlen and Neukieritzsch. In 2014, these supplemental district heating services raised the overall fuel use efficiency at Lippendorf to 44.07%.
NEW HARD-COAL POWER STATION BEGINS DISTRICT HEATING
An example of a newly built CHP plant is the hard-coal-fired power station at Lünen, in North Rhine-Westphalia. This plant was recently profiled by Cornerstone because it boasts one of the highest electrical efficiencies of any power plant in the world.1 Construction of the plant was fully completed in 2014 by Trianel GmbH. The original electricity-only configuration with a generation capacity of 750 MW had already achieved an efficiency of 45.95%. Since November 2014, plant operation has been modified so that up to 35 MW of excess heat are being fed through a 617-m pipeline into the Stadtwerke Lünen utility heating network. This thermal energy from the plant covers 93% of the district heating needs of Lünen—a medium-sized city with 85,000 inhabitants. The remaining thermal energy for district heating comes from several small-scale biogas plants.
Through the conversion from electricity-only to CHP, the utilization efficiency of the hard coal has been improved. Using the 35 MW of formerly wasted heat increased the overall fuel utilization efficiency of the power plant to 47.51%. The associated reduction in electrical efficiency is reduced only insignificantly to 44.96%, the electrical capacity to 736 MW.
In principle, the power plant steam turbine system could provide up to 160 MW of district heat. Trianel is negotiating with additional potential customers for such heating services. The company estimates that the total fuel utilization efficiency would exceed 50% if the full 160 MW of heat were being provided.
Trianel has not stated what investments have been needed to establish waste heat utilization and distribution. The company estimates a payback time well in excess of 10 years for these expenditures.
VERSATILE LIGNITE POWER AND HEAT GENERATION IN A SUBURBAN SETTING
The three lignite units comprising the CHP facility in Chemnitz, Saxony, were originally constructed from 1986 to 1990 at the north end of the city. The municipal utility Stadtwerke Chemnitz undertook comprehensive environmental retrofits between 1995 and 1998. The original lignite-fired Unit A was converted to run on natural gas and light heating oil. Lignite-fired boilers B and C were retrofitted with desulfurization equipment. The crude lignite is delivered by rail from an opencast mine about 70 km away.
Both lignite-fired units are operated in a CHP configuration. Unlike the plants described previously, more than half of the plant energy output is used for heating rather than power production. Unit C operates throughout the year to generate 100 MW of electricity with a heating capacity of 140 MW. With its extraction-condensation turbine, it can provide varying levels of power generation as required to meet fluctuating demand. Similarly, waste heat is supplied as needed. Unit B operates exclusively in the cooler months of the year, delivering 67 MW of electricity and 165 MW of heat. Similar capacities are provided by Unit A, which is employed as a backup unit.
The Chemnitz CHP facility generates enough electricity for all 140,000 private households in the city and provides heat for about one third of the households. The current operator, Eins Energie (which succeeded Stadtwerke Chemnitz), declared an achieved electrical efficiency of 24.3% on average in 2014—notably lower than the electrical efficiency of the newer and larger plants. However, the overall fuel use efficiency was 58.8%, a figure that was reduced because Unit C generates electricity during the warmer times of the year when there is no commensurate demand for district heat.
Acceptance by the local community is critical for the success of CHP plants sited near residential areas. In the experience of the operator of the Chemnitz facility, the majority of the local population accepts the use of lignite and the plant. At one time, residents near the plant repeatedly complained about the noise level of the freight yard operations; more recently, the plant operator has worked to ensure that such disturbances are minimized.
THE SMALL, EFFICIENT BAUTZEN CHP PLANT
Dedicated in 1980 as a heating-only facility, the CHP power plant in Bautzen, Saxony, was reconfigured for CHP operation in 1995 with the required environmental control technologies. Pulverized lignite is now the primary fuel, delivered by tank trucks and discharged pneumatically into storage silos at the plant site.
One of the two boilers supplying the back-pressure turbine is fired with the finely ground lignite. If required, the second boiler can be added with natural gas or heating oil. The CHP plant primarily delivers 40 MW of heating energy, while electrical generation lies at 2 MW. The operator Enso reported an electrical efficiency of 19% and overall fuel use efficiency of 78%. Lying on the outskirts of the medium-sized city of 40,000, this CHP facility delivers heat to the municipal utility Energie- und Wasserwerke Bautzen.
Enso has employed the pulverized lignite-based CHP successfully for the last 20 years. It functions reliably and is accepted by the local inhabitants, even though a suburban housing complex sits only 250 m from the plant site. One reason is that many Bautzen residents are involved with the local lignite industry, either directly or indirectly.
Continued development of the pulverized lignite CHP technology used in Bautzen began in the 1990s by the regional mining company, Laubag. At that time, many eastern German cities were converting their older lignite-fired CHP plants to modern natural-gas-fired facilities, leading to a significant sales reduction for Laubag. The company had hoped that cities would prefer advanced lignite technologies as the basis of electricity and heating services. However, this expectation has only been partially fulfilled. Laubag’s successor, Vattenfall Europe Mining, currently supplies 10 municipal CHP plants with pulverized lignite. The most recent plant was converted to use this processed domestic fuel in 2010.
EFFICIENT ENERGY THROUGH COGENERATION
Whether large or small, urban centers provide an opportunity for greater deployment of combined heat and power production. While Germany has a long history employing CHP, this technology also offers benefits around the world. As energy demand continues to grow, it is worth considering how best to utilize precious energy resources. The flexibility and efficiency offered by CHP plants is one option that continues to carry merit.
Much of the information in this article was provided by the CHP facility operators and owners. Their contributions are gratefully acknowledged.
A. All electrical efficiencies are reported in terms of lower heating value (LHV).
- Santioanni, D. (2015). Setting the benchmark: The world’s most efficient coal-fired power plants, Cornerstone, 3(1), 39–42, cornerstonemag.net/setting-the-benchmark-the-worlds-most-efficient-coal-fired-power-plants/
To contact the author, please visit www.stefanschroeter.com
The content in Cornerstone does not necessarily reflect the views of the World Coal Association or its members.
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